Accuracy of streaming RNN transducer

ABSTRACT

A computer-implemented method is provided for model training. The method includes training a second end-to-end neural speech recognition model that has a bidirectional encoder to output same symbols from an output probability lattice of the second end-to-end neural speech recognition model as from an output probability lattice of a trained first end-to-end neural speech recognition model having a unidirectional encoder. The method also includes building a third end-to-end neural speech recognition model that has a unidirectional encoder by training the third end-to-end neural speech recognition model as a student by using the trained second end-to-end neural speech recognition model as a teacher in a knowledge distillation method.

BACKGROUND

The present invention relates generally to information processing and,in particular, to improving the accuracy of a streaming Recurrent NeuralNetwork (RNN) transducer for end-to-end speech recognition.

End-to-end training of RNN transducer (RNN-T) models does not requireframe-level alignments between audio and output symbols. As a result,corresponding nodes in the posterior lattices defined by the jointnetworks from different RNN-T models can emit different symbols, whichposes a new set of challenges in knowledge distillation between RNN-Tmodels. In particular, these discrepancies in the posterior lattices areprominent between an offline RNN-T model and a streaming RNN-T modelbecause the streaming RNN-T model emits symbols later than the offlineRNN-T model.

SUMMARY

According to aspects of the present invention, a computer-implementedmethod is provided for model training. The method includes training asecond end-to-end neural speech recognition model that has abidirectional encoder to output same symbols from an output probabilitylattice of the second end-to-end neural speech recognition model as froman output probability lattice of a trained first end-to-end neuralspeech recognition model having a unidirectional encoder. The methodalso includes building a third end-to-end neural speech recognitionmodel that has a unidirectional encoder by training the third end-to-endneural speech recognition model as a student by using the trained secondend-to-end neural speech recognition model as a teacher in a knowledgedistillation method.

According to other aspects of the present invention, a computer programproduct is provided for model training. The computer program productincludes a non-transitory computer readable storage medium havingprogram instructions embodied therewith, the program instructionsexecutable by a computer to cause the computer to perform a method. Themethod includes training a second end-to-end neural speech recognitionmodel that has a bidirectional encoder to output same symbols from anoutput probability lattice of the second end-to-end neural speechrecognition model as from an output probability lattice of a trainedfirst end-to-end neural speech recognition model having a unidirectionalencoder. The method also includes building a third end-to-end neuralspeech recognition model that has a unidirectional encoder by trainingthe third end-to-end neural speech recognition model as a student byusing the trained second end-to-end neural speech recognition model as ateacher in a knowledge distillation method.

According to yet other aspects of the present invention, a computerprocessing system is provided for model training. The computerprocessing system includes a memory device for storing program code. Thecomputer processing system further includes a hardware processoroperatively coupled to the memory device for running the program code totrain a second end-to-end neural speech recognition model that has abidirectional encoder to output same symbols from an output probabilitylattice of the second end-to-end neural speech recognition model as froman output probability lattice of a trained first end-to-end neuralspeech recognition model having a unidirectional encoder. The hardwareprocessor also runs the program code to build a third end-to-end neuralspeech recognition model that has a unidirectional encoder by trainingthe third end-to-end neural speech recognition model as a student byusing the trained second end-to-end neural speech recognition model as ateacher in a knowledge distillation method.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will provide details of preferred embodimentswith reference to the following figures wherein:

FIG. 1 is a block diagram showing an exemplary computing device, inaccordance with an embodiment of the present invention;

FIGS. 2-3 show an exemplary method for improving the accuracy of astreaming RNN transducer for end-to-end speech recognition, inaccordance with an embodiment of the present invention;

FIG. 4 shows an exemplary unidirectional RNN-T architecture, inaccordance with an embodiment of the present invention;

FIG. 5 shows an exemplary bi-directional RNN-T architecture, inaccordance with an embodiment of the present invention;

FIG. 6 is a block diagram further showing the output probability latticeof FIGS. 4 and 5 , in accordance with an embodiment of the presentinvention;

FIG. 7 is a block diagram graphically showing the elements involved in ablock of method 200 of FIG. 2 , in accordance with an embodiment of thepresent invention;

FIG. 8 is a block diagram graphically showing the elements involved inanother block of method 200 of FIG. 2 , in accordance with an embodimentof the present invention;

FIG. 9 is a block diagram graphically showing the elements involved inyet another block of method 200 of FIG. 2 , in accordance with anembodiment of the present invention;

FIG. 10 is a block diagram further graphically showing the RNN-Telements involved in a block of FIG. 2 , in accordance with anembodiment of the present invention;

FIG. 11 is a block diagram showing an illustrative cloud computingenvironment having one or more cloud computing nodes with which localcomputing devices used by cloud consumers communicate, in accordancewith an embodiment of the present invention; and

FIG. 12 is a block diagram showing a set of functional abstractionlayers provided by a cloud computing environment, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention are directed to improving theaccuracy of a streaming Recurrent Neural Network (RNN) transducer forend-to-end speech recognition.

In accordance with an embodiment of the present invention, a method isproposed to train a RNN-T model so that the nodes of its posteriorlattice emit the same symbols with the corresponding nodes of theposterior lattice from the pre-trained RNN-T model. By utilizing thismethod, an offline RNN-T model can be trained that can serve as a goodteacher to train a student streaming RNN-T model.

It is to be appreciated that while RNN-T models are primarily describedin the embodiments set forth herein, other models including, forexample, but not limited to, transformer transducers, RNN-transducerwith stateless prediction network, and so forth.

FIG. 1 is a block diagram showing an exemplary computing device 100, inaccordance with an embodiment of the present invention. The computingdevice 100 is configured to improve the accuracy of a streaming RNNtransducer model for end-to-end speech recognition.

The computing device 100 may be embodied as any type of computation orcomputer device capable of performing the functions described herein,including, without limitation, a computer, a server, a rack basedserver, a blade server, a workstation, a desktop computer, a laptopcomputer, a notebook computer, a tablet computer, a mobile computingdevice, a wearable computing device, a network appliance, a webappliance, a distributed computing system, a processor-based system,and/or a consumer electronic device. Additionally or alternatively, thecomputing device 100 may be embodied as a one or more compute sleds,memory sleds, or other racks, sleds, computing chassis, or othercomponents of a physically disaggregated computing device. As shown inFIG. 1 , the computing device 100 illustratively includes the processor110, an input/output subsystem 120, a memory 130, a data storage device140, and a communication subsystem 150, and/or other components anddevices commonly found in a server or similar computing device. Ofcourse, the computing device 100 may include other or additionalcomponents, such as those commonly found in a server computer (e.g.,various input/output devices), in other embodiments. Additionally, insome embodiments, one or more of the illustrative components may beincorporated in, or otherwise form a portion of, another component. Forexample, the memory 130, or portions thereof, may be incorporated in theprocessor 110 in some embodiments.

The processor 110 may be embodied as any type of processor capable ofperforming the functions described herein. The processor 110 may beembodied as a single processor, multiple processors, a CentralProcessing Unit(s) (CPU(s)), a Graphics Processing Unit(s) (GPU(s)), asingle or multi-core processor(s), a digital signal processor(s), amicrocontroller(s), or other processor(s) or processing/controllingcircuit(s).

The memory 130 may be embodied as any type of volatile or non-volatilememory or data storage capable of performing the functions describedherein. In operation, the memory 130 may store various data and softwareused during operation of the computing device 100, such as operatingsystems, applications, programs, libraries, and drivers. The memory 130is communicatively coupled to the processor 110 via the I/O subsystem120, which may be embodied as circuitry and/or components to facilitateinput/output operations with the processor 110 the memory 130, and othercomponents of the computing device 100. For example, the I/O subsystem120 may be embodied as, or otherwise include, memory controller hubs,input/output control hubs, platform controller hubs, integrated controlcircuitry, firmware devices, communication links (e.g., point-to-pointlinks, bus links, wires, cables, light guides, printed circuit boardtraces, etc.) and/or other components and subsystems to facilitate theinput/output operations. In some embodiments, the I/O subsystem 120 mayform a portion of a system-on-a-chip (SOC) and be incorporated, alongwith the processor 110, the memory 130, and other components of thecomputing device 100, on a single integrated circuit chip.

The data storage device 140 may be embodied as any type of device ordevices configured for short-term or long-term storage of data such as,for example, memory devices and circuits, memory cards, hard diskdrives, solid state drives, or other data storage devices. The datastorage device 140 can store program code for (improving the accuracyof) a streaming RNN transducer model(s) for end-to-end speechrecognition. The communication subsystem 150 of the computing device 100may be embodied as any network interface controller or othercommunication circuit, device, or collection thereof, capable ofenabling communications between the computing device 100 and otherremote devices over a network. The communication subsystem 150 may beconfigured to use any one or more communication technology (e.g., wiredor wireless communications) and associated protocols (e.g., Ethernet,InfiniBand®, Bluetooth®, Wi-Fi®, WiMAX, etc.) to effect suchcommunication.

As shown, the computing device 100 may also include one or moreperipheral devices 160. The peripheral devices 160 may include anynumber of additional input/output devices, interface devices, and/orother peripheral devices. For example, in some embodiments, theperipheral devices 160 may include a display, touch screen, graphicscircuitry, keyboard, mouse, speaker system, microphone, networkinterface, and/or other input/output devices, interface devices, and/orperipheral devices.

Of course, the computing device 100 may also include other elements (notshown), as readily contemplated by one of skill in the art, as well asomit certain elements. For example, various other input devices and/oroutput devices can be included in computing device 100, depending uponthe particular implementation of the same, as readily understood by oneof ordinary skill in the art. For example, various types of wirelessand/or wired input and/or output devices can be used. Moreover,additional processors, controllers, memories, and so forth, in variousconfigurations can also be utilized. Further, in another embodiment, acloud configuration can be used (e.g., see FIGS. 11-12 ). These andother variations of the processing system 100 are readily contemplatedby one of ordinary skill in the art given the teachings of the presentinvention provided herein.

As employed herein, the term “hardware processor subsystem” or “hardwareprocessor” can refer to a processor, memory (including RAM, cache(s),and so forth), software (including memory management software) orcombinations thereof that cooperate to perform one or more specifictasks. In useful embodiments, the hardware processor subsystem caninclude one or more data processing elements (e.g., logic circuits,processing circuits, instruction execution devices, etc.). The one ormore data processing elements can be included in a central processingunit, a graphics processing unit, and/or a separate processor- orcomputing element-based controller (e.g., logic gates, etc.). Thehardware processor subsystem can include one or more on-board memories(e.g., caches, dedicated memory arrays, read only memory, etc.). In someembodiments, the hardware processor subsystem can include one or morememories that can be on or off board or that can be dedicated for use bythe hardware processor subsystem (e.g., ROM, RAM, basic input/outputsystem (BIOS), etc.).

In some embodiments, the hardware processor subsystem can include andexecute one or more software elements. The one or more software elementscan include an operating system and/or one or more applications and/orspecific code to achieve a specified result.

In other embodiments, the hardware processor subsystem can includededicated, specialized circuitry that performs one or more electronicprocessing functions to achieve a specified result. Such circuitry caninclude one or more application-specific integrated circuits (ASICs),FPGAs, and/or PLAs.

These and other variations of a hardware processor subsystem are alsocontemplated in accordance with embodiments of the present invention

FIGS. 2-3 show an exemplary method 200 for improving the accuracy of astreaming RNN transducer for end-to-end speech recognition, inaccordance with an embodiment of the present invention.

At block 210, train a first end-to-end neural speech recognition modelthat has a unidirectional encoder. In an embodiment, the firstend-to-end neural speech recognition model can be considered a referenceend-to-end neural speech recognition model.

At block 220, train a second end-to-end neural speech recognition modelthat has a bidirectional encoder to output same symbols from an outputprobability lattice of the second end-to-end neural speech recognitionmodel as from an output probability lattice of the trained firstend-to-end neural speech recognition model. In an embodiment, thetraining can be performed with respect to an end-to-end neural speechrecognition model loss (RNN-T loss) and a cross entropy loss. In anembodiment, the end-to-end neural speech recognition model loss isobtained by referring to labels in the training data. In an embodiment,the cross entropy loss is calculated using the 1-best output symbolsfrom the unidirectional model trained in block 210. In an embodiment,each of the nodes of the output probability lattice can represent asoftmax operation. In an embodiment, the second end-to-end neural speechrecognition model comprises a stronger or more complex configuration(e.g., a deeper neural network) than the first end-to-end neural speechrecognition model.

In an embodiment, block 220 can include one or more of blocks 220A and220B.

At block 220A, obtain a 1-best symbol for each of nodes of the outputprobability lattice. This is achieved by feeding the training audio datato the first reference end-to-end speech recognition model and selectingthe 1-best symbol from each node in the output posterior lattice.

At block 220B, minimize an end-to-end neural speech recognition modelloss and also a cross entropy loss regarding the 1-best symbols.

At 220C, minimize the end-to-end neural speech recognition model lossand the cross entropy loss until the two losses become sufficientlysmall, e.g., below respective thresholds.

At block 230, train a third end-to-end neural speech recognition modelthat has a unidirectional encoder as a student by using the trainedsecond end-to-end neural speech recognition model as a teacher in aknowledge distillation method. In an embodiment, the third end-to-endneural speech recognition system can be trained until the similaritybetween the output probability lattices of the third and secondend-to-end neural speech recognition models is within a particularsimilarity range.

In an embodiment, block 230 includes one or more of blocks 230A, 230B,230C, and 230D.

At block 230A, input training audio data into the trained secondend-to-end neural speech recognition model to obtain the outputprobability lattice therefor.

At block 230B, mask less probable nodes of the output probabilitylattice with respect to a minimum probability threshold.

At block 230C, jointly minimize (i) the end-to-end neural speechrecognition model loss and (ii) a relative divergence over unmaskedportions of the output probability lattice to an output probabilitylattice of the third end-to-end speech recognition model. The relativedivergence can be a Kullback-Leibler divergence or some otherdivergence.

At block 230D, evaluate the output probability lattice of the thirdend-to-end neural speech recognition model based on a similarity to anoutput probability lattice of the second end-to-end speech recognitionsystem. For example, similarity can be based on KL divergence, and/or soforth.

At block 240, perform speech recognition by performing a beam searchover an output probability lattice of the trained third end-to-endneural speech recognition model.

In an embodiment, the first and third end-to-end neural speechrecognition models stream output data from a RNN transducer, and thesecond end-to-end neural speech recognition model provides the outputdata offline.

The first end-to-end neural speech recognition model can be considered areference model. The second end-to-end neural speech recognition modelcan be considered as a teacher model. The third end-to-end neural speechrecognition model can be considered as a student model.

Each of the first, second, and third end-to-end neural speechrecognition models employ a Recurrent Neural Network Transducer (RNN-T)architecture. The first and third end-to-end neural speech recognitionmodels are unidirectional, while the second end-to-end neural speechrecognition model is bi-directional. The architectures are furtherdescribed hereinbelow.

FIG. 4 shows an exemplary unidirectional RNN-T architecture 400, inaccordance with an embodiment of the present invention.

The unidirectional RNN-T architecture 400 includes a prediction network410, a unidirectional encoder block 420, a joint network 430, a softmaxblock 440, and an output probability lattice 450.

The unidirectional encoder block 420 receives an input feature sequencex=(x₁, . . . , x_(T)).

Based on a search over an output probability lattice defined byP(y|t,u), the unidirectional RNN-T architecture 400 generates an outputsequence y. In an embodiment, y is a set of possible symbols, t is atime index, and u is a history of emitted symbols.

In detail, the unidirectional encoder block 420 receives the inputfeature sequence x=(x₁, . . . , x_(T)), and outputs an encoding h_(t)^(enc).

The prediction network 410 receives a prior prediction y_(u)−1 andoutputs a prediction h_(u) ^(pred).

The joint network 430 receives h_(t) ^(enc) and h_(u) ^(pred) andoutputs z_(t,u).

The softmax block receives z_(t,u) and a softmax of z_(t,u).

The output probability lattice 450, defined by P(y|t,u), receives thesoftmax of z_(t,u) and outputs sequence y=(y₁, . . . , y_(u)).

FIG. 5 shows an exemplary bi-directional RNN-T architecture 500, inaccordance with an embodiment of the present invention.

The RNN-T architecture 500 includes a prediction network 510, abidirectional encoder block 520, a joint network 530, a softmax block540, and an output probability lattice 550.

FIG. 6 is a block diagram further showing the output probability latticeof FIGS. 4 and 5 , in accordance with an embodiment of the presentinvention.

The output probability lattice 450/550 can be considered relative to anx axis and a y axis such that x=(x₁, . . . , x_(T)) and y=(y₁, . . . ,y_(U)).

Each node in the output probability lattice 450/550 represents a softmaxof z_(t,u).

Speech recognition with the RNN-T architecture 400 is realized by a beamsearch over the output probability lattice 450/550.

FIG. 7 is a block diagram graphically showing the elements involved inblock 210 of method 200 of FIG. 2 , in accordance with an embodiment ofthe present invention.

The involved elements in block 210 are a training data database 710, anda unidirectional RNN-T 720. The unidirectional RNN-T 720 can be formedto have a similar architecture to RNN-T architecture 300 of FIG. 3 .

FIG. 8 is a block diagram graphically showing the elements involved inblock 220 of method 200 of FIG. 2 , in accordance with an embodiment ofthe present invention.

The involved elements in block 220 are the training data database 710and the unidirectional RNN-T 720, as well as the bi-directional RNN-T730. The bidirectional RNN-T 720 can be formed to have a similararchitecture to RNN-T architecture 500 of FIG. 5 .

The 1-best symbol for each node of the output probability lattice isoutput from the unidirectional RNN-T 720. This is used to train thebidirectional RNN-T 730.

FIG. 9 is a block diagram graphically showing the elements involved inblock 230 of method 200 of FIG. 2 , in accordance with an embodiment ofthe present invention.

The involved elements in block 230 are the training data database 710,the unidirectional RNN-T 720, the bi-directional RNN-T 730, as well asanother unidirectional RNN-T 740. The unidirectional RNN-T 740 can beformed to have a similar architecture to RNN-T architecture 300 of FIG.3 .

The output from the trained bidirectional RNN-T (730) is used to trainthe unidirectional RNN-T (at the bottom). “Posterior” is computed foreach node in the output lattice from 730.

FIG. 10 is a block diagram further graphically showing the RNN-Telements involved in block 220 of FIG. 2 , in accordance with anembodiment of the present invention.

The involved elements in block 220 are training data database 1010,unidirectional RNN-T 1020, and bi-directional RNN-T 1030.

The unidirectional RNN-T 1020, which can be considered a referenceRNN-T, includes a unidirectional prediction network 1020A, aunidirectional encoder network 1020B, and an output probability lattice1020C.

The bidirectional RNN-T 1030, which can be considered a teacher RNN-T,includes a deep prediction network 1030A, a bidirectional/deep encodernetwork 1030B, and an output probability lattice 1030C.

Reference numeral 1020C denotes output lattice from the reference model,while 1020D is the one-best selected from output lattice 1020C. Hence,the same model 1020 is involved on the right side of FIG. 10 , justshown essentially at different time steps in relation to obtain the 1best outputs.

It is to be appreciated that while not shown in FIG. 10 , some of theblocks of the output probability lattices can be masked. In anembodiment, masked blocks are ignored in the training of models. Thismasking is used when training the third network by referring the secondnetwork.

Regarding operation, the unidirectional RNN-T 1020 and the bidirectionalRNN-T 1030 receive training samples from the training data database1010. The unidirectional RNN-T 1020 selects 1-best for each node of theoutput probability lattice 1020C. Cross entropy is minimized between thebidirectional RNN-T 1030 and the unidirectional RNN-T 1040.

It is to be understood that although this disclosure includes a detaileddescription on cloud computing, implementation of the teachings recitedherein are not limited to a cloud computing environment. Rather,embodiments of the present invention are capable of being implemented inconjunction with any other type of computing environment now known orlater developed.

Cloud computing is a model of service delivery for enabling convenient,on-demand network access to a shared pool of configurable computingresources (e.g., networks, network bandwidth, servers, processing,memory, storage, applications, virtual machines, and services) that canbe rapidly provisioned and released with minimal management effort orinteraction with a provider of the service. This cloud model may includeat least five characteristics, at least three service models, and atleast four deployment models.

Characteristics are as follows:

On-demand self-service: a cloud consumer can unilaterally provisioncomputing capabilities, such as server time and network storage, asneeded automatically without requiring human interaction with theservice's provider.

Broad network access: capabilities are available over a network andaccessed through standard mechanisms that promote use by heterogeneousthin or thick client platforms (e.g., mobile phones, laptops, and PDAs).

Resource pooling: the provider's computing resources are pooled to servemultiple consumers using a multi-tenant model, with different physicaland virtual resources dynamically assigned and reassigned according todemand. There is a sense of location independence in that the consumergenerally has no control or knowledge over the exact location of theprovided resources but may be able to specify location at a higher levelof abstraction (e.g., country, state, or datacenter).

Rapid elasticity: capabilities can be rapidly and elasticallyprovisioned, in some cases automatically, to quickly scale out andrapidly released to quickly scale in. To the consumer, the capabilitiesavailable for provisioning often appear to be unlimited and can bepurchased in any quantity at any time.

Measured service: cloud systems automatically control and optimizeresource use by leveraging a metering capability at some level ofabstraction appropriate to the type of service (e.g., storage,processing, bandwidth, and active user accounts). Resource usage can bemonitored, controlled, and reported, providing transparency for both theprovider and consumer of the utilized service.

Service Models are as follows:

Software as a Service (SaaS): the capability provided to the consumer isto use the provider's applications running on a cloud infrastructure.The applications are accessible from various client devices through athin client interface such as a web browser (e.g., web-based e-mail).The consumer does not manage or control the underlying cloudinfrastructure including network, servers, operating systems, storage,or even individual application capabilities, with the possible exceptionof limited user-specific application configuration settings.

Platform as a Service (PaaS): the capability provided to the consumer isto deploy onto the cloud infrastructure consumer-created or acquiredapplications created using programming languages and tools supported bythe provider. The consumer does not manage or control the underlyingcloud infrastructure including networks, servers, operating systems, orstorage, but has control over the deployed applications and possiblyapplication hosting environment configurations.

Infrastructure as a Service (IaaS): the capability provided to theconsumer is to provision processing, storage, networks, and otherfundamental computing resources where the consumer is able to deploy andrun arbitrary software, which can include operating systems andapplications. The consumer does not manage or control the underlyingcloud infrastructure but has control over operating systems, storage,deployed applications, and possibly limited control of select networkingcomponents (e.g., host firewalls).

Deployment Models are as follows:

Private cloud: the cloud infrastructure is operated solely for anorganization. It may be managed by the organization or a third party andmay exist on-premises or off-premises.

Community cloud: the cloud infrastructure is shared by severalorganizations and supports a specific community that has shared concerns(e.g., mission, security requirements, policy, and complianceconsiderations). It may be managed by the organizations or a third partyand may exist on-premises or off-premises.

Public cloud: the cloud infrastructure is made available to the generalpublic or a large industry group and is owned by an organization sellingcloud services.

Hybrid cloud: the cloud infrastructure is a composition of two or moreclouds (private, community, or public) that remain unique entities butare bound together by standardized or proprietary technology thatenables data and application portability (e.g., cloud bursting forload-balancing between clouds).

A cloud computing environment is service oriented with a focus onstatelessness, low coupling, modularity, and semantic interoperability.At the heart of cloud computing is an infrastructure that includes anetwork of interconnected nodes.

Referring now to FIG. 11 , illustrative cloud computing environment 1150is depicted. As shown, cloud computing environment 1150 includes one ormore cloud computing nodes 1110 with which local computing devices usedby cloud consumers, such as, for example, personal digital assistant(PDA) or cellular telephone 1154A, desktop computer 1154B, laptopcomputer 1154C, and/or automobile computer system 1154N may communicate.Nodes 1110 may communicate with one another. They may be grouped (notshown) physically or virtually, in one or more networks, such asPrivate, Community, Public, or Hybrid clouds as described hereinabove,or a combination thereof. This allows cloud computing environment 1150to offer infrastructure, platforms and/or software as services for whicha cloud consumer does not need to maintain resources on a localcomputing device. It is understood that the types of computing devices1154A-N shown in FIG. 11 are intended to be illustrative only and thatcomputing nodes 1110 and cloud computing environment 1150 cancommunicate with any type of computerized device over any type ofnetwork and/or network addressable connection (e.g., using a webbrowser).

Referring now to FIG. 12 , a set of functional abstraction layersprovided by cloud computing environment 1150 (FIG. 11 ) is shown. Itshould be understood in advance that the components, layers, andfunctions shown in FIG. 12 are intended to be illustrative only andembodiments of the invention are not limited thereto. As depicted, thefollowing layers and corresponding functions are provided:

Hardware and software layer 1260 includes hardware and softwarecomponents. Examples of hardware components include: mainframes 1261;RISC (Reduced Instruction Set Computer) architecture based servers 1262;servers 1263; blade servers 1264; storage devices 1265; and networks andnetworking components 1266. In some embodiments, software componentsinclude network application server software 1267 and database software1268.

Virtualization layer 1270 provides an abstraction layer from which thefollowing examples of virtual entities may be provided: virtual servers1271; virtual storage 1272; virtual networks 1273, including virtualprivate networks; virtual applications and operating systems 1274; andvirtual clients 1275.

In one example, management layer 1280 may provide the functionsdescribed below. Resource provisioning 1281 provides dynamic procurementof computing resources and other resources that are utilized to performtasks within the cloud computing environment. Metering and Pricing 1282provide cost tracking as resources are utilized within the cloudcomputing environment, and billing or invoicing for consumption of theseresources. In one example, these resources may include applicationsoftware licenses. Security provides identity verification for cloudconsumers and tasks, as well as protection for data and other resources.User portal 1283 provides access to the cloud computing environment forconsumers and system administrators. Service level management 1284provides cloud computing resource allocation and management such thatrequired service levels are met. Service Level Agreement (SLA) planningand fulfillment 1285 provide pre-arrangement for, and procurement of,cloud computing resources for which a future requirement is anticipatedin accordance with an SLA.

Workloads layer 1290 provides examples of functionality for which thecloud computing environment may be utilized. Examples of workloads andfunctions which may be provided from this layer include: mapping andnavigation 1291; software development and lifecycle management 1292;virtual classroom education delivery 1293; data analytics processing1294; transaction processing 1295; and accuracy increase in RNN-T forend-to-end speech recognition 1296.

The present invention may be a system, a method, and/or a computerprogram product at any possible technical detail level of integration.The computer program product may include a computer readable storagemedium (or media) having computer readable program instructions thereonfor causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as SMALLTALK, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present invention, as well as other variations thereof, means that aparticular feature, structure, characteristic, and so forth described inconnection with the embodiment is included in at least one embodiment ofthe present invention. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Having described preferred embodiments of a system and method (which areintended to be illustrative and not limiting), it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments disclosed which arewithin the scope of the invention as outlined by the appended claims.Having thus described aspects of the invention, with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

The invention claimed is:
 1. A computer-implemented method for modeltraining, comprising: training a second end-to-end neural speechrecognition model that has a bidirectional encoder to output samesymbols from an output probability lattice of the second end-to-endneural speech recognition model as from an output probability lattice ofa trained first end-to-end neural speech recognition model haying aunidirectional encoder; and building a third end-to-end neural speechrecognition model that has a unidirectional encoder by training thethird end-to-end speech recognition model as a student by using thetrained second end-to-end neural speech recognition model as a teacherin a knowledge distillation method, the first and third end-to-endneural speech recognition models being configured to stream output datafrom a recurrent neural network transducer (RNN-T), and the secondend-to-end neural speech recognition model provides the output dataoffline.
 2. The computer-implemented method of claim 1, wherein trainingthe second end-to-end speech recognition model includes: obtaining a1-best symbol for each of nodes of the output probability lattice of thefirst end-to-end neural speech recognition model by feeding trainingaudio data to the first reference end-to-end speech recognition model,selecting the 1-best symbol from each node in the output posteriorlattice, and outputting the 1 best symbol for each node from theunidirectional encoder for training the bidirectional encoder; andminimizing an end-to-end neural speech recognition model loss and also across entropy loss regarding the 1-best symbol.
 3. Thecomputer-implemented method of claim 2, wherein a weighted summation ofthe cross entropy loss and the end-to-end speech recognition model lossis minimized.
 4. The computer-implemented method of claim 2, whereintraining the third end-to-end neural speech recognition model includes:masking less probable nodes of the output probability lattice of thesecond end-to-end neural speech recognition model with respect to aminimum probability threshold; and jointly minimizing (i) the end-to-endneural speech recognition model loss and (ii) a relative divergence overunmasked portions of the output probability lattice of the secondend-to-end neural speech recognition model to an output probabilitylattice of the third end-to-end speech recognition model.
 5. Thecomputer-implemented method of claim 4, wherein the relative divergenceis a Kullback-Leibler divergence.
 6. The computer-implemented method ofclaim 1, wherein training the second end-to-end speech recognition modelfurther includes inputting training audio data into the trained firstend-to-end neural speech recognition model to obtain the outputprobability lattice of the second end-to-end neural speech recognitionmodel.
 7. The computer-implemented method of claim 1, wherein each ofthe nodes of the output probability lattice of the second end-to-endneural speech recognition. model represent a softmax operation.
 8. Thecomputer-implemented method of claim 1, further comprising performingspeech recognition by performing a beam search over an outputprobability lattice of the trained third end-to-end neural speechrecognition model.
 9. The computer-implemented method of claim 1,wherein the second end-to-end neural speech recognition model comprisesa more complex neural network than the first end-to-end neural speechrecognition model.
 10. The computer-implemented method of claim 1,thither comprising evaluating the output probability lattice of thethird end-to-end neural speech recognition model based on a similarityto the output probability lattice of the second end-to-end speechrecognition system.
 11. The computer-implemented method of claim 1,wherein the third end- to-end neural speech recognition system istrained until the similarity between the output probability lattices ofthe third and second end-to-end neural speech recognition models iswithin a particular similarity range.
 12. The computer-implementedmethod of claim 1, wherein the method is performed by a speechrecognition system.
 13. The computer-implemented method of claim 1,wherein at least one of the first, second and third neural speechrecognition models comprise a Recurrent Neural Network Transducer model.14. The computer-implemented method of claim 1, wherein the secondend-to-end neural speech recognition model comprises an ensemble ofend-to-end neural speech recognition models, and the method furthercomprises selecting a best teacher model from the ensemble responsive toan overlap of search paths through an output probability lattice of eachof the end-to-end neural speech recognition models in the ensemble. 15.A computer program product for model training, the computer programproduct comprising a non-transitory computer readable storage mediumhaving program instructions embodied therewith, the program instructionsexecutable by a computer to cause the computer to perform a methodcomprising: training a second end-to-end neural speech recognition modelthat has a bidirectional encoder to output same symbols from an outputprobability lattice of the second end-to-end neural speech recognitionmodel as from an output probability lattice of a trained firstend-to-end neural speech recognition model having, a unidirectionalencoder; and building a third end-to-end neural speech recognition modelthat has a unidirectional encoder by training the third end-to-endspeech recognition model as a student by using the trained secondend-to-end neural speech recognition model as a teacher in a knowledgedistillation method, the first and third end-to-end neural speechrecognition models being configured to stream output data from arecurrent neural network transducer (RNN-T), and the second end-to-endneural speech recognition model provides the output data offline. 16.The computer-implemented method of claim 15, wherein training the secondend-to-end speech recognition model includes: obtaining a 1-best symbolfor each of nodes of the output probability lattice of the firstend-to-end neural speech recognition model by feeding training audiodata to the first reference end-to-end speech recognition model,selecting the 1-best symbol from each node in the output posteriorlattice, and outputting the 1 best symbol for each node from theunidirectional encoder for training the bidirectional encoder; andminimizing an end-to-end neural speech recognition model loss and also across entropy loss regarding the 1-best symbol.
 17. Thecomputer-implemented method of claim 16, wherein a weighted summation ofthe cross entropy loss and the end-to-end speech recognition model lossis minimized.
 18. The computer-implemented method of claim 16, whereintraining the third end-to-end neural speech recognition model includes:masking less probable nodes of the output probability lattice of thesecond end-to-end neural speech recognition model with respect to aminimum probability threshold; and jointly minimizing (i) the end-to-endneural speech recognition model loss and (ii) a relative divergence overunmasked portions of the output probability lattice of the secondend-to-end neural speech recognition model to an output probabilitylattice of the third end-to-end speech recognition model.
 19. A computerprocessing system for model training, comprising: a memory device forstoring program code; and a hardware processor operatively coupled tothe memory device for running the program code to; train a secondend-to-end neural speech recognition model that has a bidirectionalencoder to output same symbols from an output probability lattice of thesecond end-to-end neural speech recognition model as from an outputprobability lattice of a trained first end-to-end neural speechrecognition model having a unidirectional encoder; and build a thirdend-to-end neural speech recognition model that has a unidirectionalencoder by training the third end-to-end speech recognition model as astudent by using the trained second end-to-end neural speech recognitionmodel as a teacher in a knowledge distillation method, the first andthird end-to-end neural speech recognition models being configured tostream output data from a recurrent neural network transducer (RNN-T),and the second end-to-end neural speech recognition model provides theoutput data offline.